This invention relates to a mass flow sensor utilized for measurement of mass flow of a gas or other fluid used in semiconductor producing processes.
Two types of mass flow meters have been heretofore known, one of which is a constant current type sensor represented typically by, for example, U.S. Pat. No. 3,938,384 and the other is a constant temperature type sensor disclosed by U.S. Pat. No. 4,464,932, Japanese Patent Publication No. 16128/83 and U.S. Pat. No. 4,815,280.
An example of the known constant current type sensor is shown in FIG. 1, in which a fluid flows in a pipe in the direction of arrow X. Heating resistors R.sub.1 and R.sub.2 are disposed about the pipe on the downstream and upstream sides, respectively, and are provided with a constant current I by a constant current source 901. Here, since voltages V.sub.1 and V.sub.2 are generated at the heating resistors R.sub.1 and R.sub.2, respectively, the difference thereof (V.sub.1 -V.sub.2) is taken out from a differential amplifier 902 through a bridge circuit shown in FIG. 1 for detection of the mass flow. Since the mass flow Q corresponds to an electric displacement generated in the heating resistors R.sub.1 and R.sub.2 when a fluid flows therethrough, the mass flow Q can be directly detected by the following formula: EQU Q.varies..DELTA.V.multidot.I.varies..DELTA.V
In contrast to this, in the constant temperature type sensor, an example of which is shown in FIG. 2, a pipe through which a fluid flows in the direction of arrow X is provided thereabout with heating resistors R.sub.1a and R.sub.1b on the downstream and upstream sides, respectively, to which an electric current is supplied through transistors T.sub.1 and T.sub.2, respectively. The heating resistors R.sub.1a and R.sub.1b in combination with resistances R.sub.2a, R.sub.3a, R.sub.4a and R.sub.2b, R.sub.3b, R.sub.4b, respectively, form bridge circuits, respectively. In each of these bridge circuits, a difference between voltages taken out at two points thereof is obtained by a comparator 911 or 912 and used to control the base currents of the transistors T.sub.1 and T.sub.2 so as to balance the bridge circuits. In other words, the control is performed to make the resistance values of the heating resistors R.sub.1a and R.sub.1b constant. As a result, the temperature of the heating resistors R.sub.1a and R.sub.1b is maintained at a predetermined value irrespective of the flow of the fluid. Here, the mass flow Q corresponds to an electric displacement generated in the heating resistors R.sub.1a and R.sub.1b when the fluid flows therethrough and is represented by the following formula: ##EQU1##
In the two types of known mass flow sensor described above, the constant current type sensor has two characteristic features. Firstly, since the current flowing through the heating resistors R.sub.1 and R.sub.2 is constant, the temperature of the heat generated by the resistors R.sub.1 and R.sub.2 changes automatically in response to the change in the ambient temperature. Accordingly, it can operate in stable condition over a wide temperature range without the necessity of using a special temperature correction circuit. Secondly, the circuit construction of this sensor is very simple. On the other hand, however, the constant current type sensor has a disadvantage in that a relatively long time is required until the temperature of the heating resistors R.sub.1 and R.sub.2 changes in response to the flowing fluid and accordingly the response is slow.
In contrast to this, the constant temperature type sensor, in which the heating resistors R.sub.1a and R.sub.1b are always maintained at a constant temperature, has a characteristic feature that its response is very quick. In fact, its response is generally ten times or more quicker than that of the constant current type sensor. On the other hand, however, the constant temperature type sensor presents a problem in that, when the ambient temperature approaches the predetermined heating temperature of the heating resistors R.sub.1a and R.sub.1b, the voltages V.sub.1 and V.sub.2 applied to the heating resistors R.sub.1a and R.sub.1b drop to make measurement difficult and when the ambient temperature exceeds the predetermined heating temperature the sensor becomes inoperable. Accordingly, it is essential that this sensor is provided with some type of a correction circuit.
Therefore, Formula (1) above can apply only when the ambient temperature and gas temperature Ta are constant. For this reason, in U.S. Pat. No. 4,815,280, it is assumed that the gas temperature Ta is proportional to (1/(V.sub.1 +V.sub.2)) and that the sensitivity decreases as the gas temperature Ta rises or the value of (1/(V.sub.1 +V.sub.2)) increases and the flow Q is obtained by the following formula: EQU Q=.DELTA.V/(V.sub.1 +V.sub.2) (2)
Even if the decrease in sensitivity is compensated for by this method, however, the range in which formula (2) above applies is narrower than that of the constant current type sensor.
Therefore, U.S. Pat. No. 4,984,460 discloses a technical art of preventing the decrease in sensitivity by means of a circuit, the principle of which is shown in FIG. 3, in which the ambient temperature detecting resistance R.sub.3b is connected in series to a heating resistor R.sub.1b of a bridge circuit comprising resistances R.sub.5b, R.sub.7b and R.sub.9b besides the heating resistor R.sub.1b and further the ambient temperature detecting resistance R.sub.4b is connected in series to a heating resistor R.sub.2b of a bridge circuit comprising resistances R.sub.6b, R.sub.8b and R.sub.10b besides the heating resistor R.sub.2b. When the ambient temperature rises, since the resistance values of the ambient temperature detecting resistances R.sub.3b and R.sub.4b are increased by means of the circuit shown in FIG. 3, control is performed so as to increase the temperature of the heating resistors R.sub.1b and R.sub.2b in response to-the amount of increase of the resistance values, to thereby prevent a decrease in sensitivity of the sensor. Here, the temperature of the heating resistance R.sub.1b is predetermined to be higher than that of the ambient temperature detecting resistance R.sub.3b by the proportion of the resistance R.sub.5b.
On the other hand, however, the proportion in resistance value of the resistance R.sub.5b to the ambient temperature detecting resistance R.sub.3b gradually decreases as the ambient temperature increases, whereby the difference between the temperature of the heating resistors R.sub.1b, R.sub.2b and the ambient temperature gradually decreases. In other words, the technical merit provided by this known technical art is such that the degree of the decrease in sensitivity caused as the temperature increases is suppressed such that the temperature is fully compensated for or the practical temperature range is substantially improved. Further, even if resistances having the same temperature coefficient and resistance value as those of the heating resistors R.sub.1b and R.sub.2b are used as the ambient temperature detecting resistances R.sub.3b and R.sub.4b, these ambient temperature detecting resistances R.sub.3b and R.sub.4b are also heated because a current having the same value as the current flowing in each of the heating resistors R.sub.1b and R.sub.2b flows in each of the ambient temperature detecting resistances R.sub.3b and R.sub.4b. This presents a problem in that the zero point of the bridge circuit becomes unstable because the resistance values of the ambient temperature detecting resistances R.sub.3b and R.sub.4b finally vary in accordance with the surrounding radiation balance.
Further, in a prior art electronic circuit, an integral matching resistance used in a plurality of independent circuits was subject to the condition that the temperature coefficient of it was very small. On the other hand, it was very difficult to bring the resistance temperature sensors into matching. Therefore, when the same temperature correction required use of a resistance temperature sensor, it had to select a plurality of resistance temperature sensor elements having the same characteristics. In an example of a prior art temperature sensing resistor shown in FIG. 4, when signals S.sub.A and S.sub.B are amplified by inversion amplifiers 71 and 72, respectively, and outputted, resistances having the same characteristics are selected as feedback resistances 83 and 84 connected to input resistances 73 and 74, respectively. The feedback resistances 83 and 84 are disposed on a base 81 provided with radiation fins 82 so that temperature correction is performed in the same environment.
In the prior art sensor shown in FIG. 4, however, it was troublesome to select resistances having the same characteristics from among a large number of resistances and, when the temperature sensing resistances were used at the same position, it was necessary to put the resistances together. Accordingly, a preintegrated temperature sensitive matching resistor has long been sought.